1. Field of the Invention
This invention relates to processes for separating from autodeposition compositions dissolved and/or dispersed metal ions having a valence of two or higher (this type of metal ions being briefly denoted hereinafter as "multivalent"), particularly iron, chromium, and/or zinc cations, more particularly iron and zinc. The invention also relates to regenerating to their acid form cation exchange resins, particularly those containing iminodiacetate (alternatively called "iminodiacetic acid"), aminophosphonic acid, and sulfonic acid functional groups, after the cation exchange resins have been at least partially loaded with multivalent metal cations, particularly with iron, chromium, and/or zinc cations. In some embodiments of particular interest, the invention relates to regenerating such ion exchange resins that have been used to remove iron, chromium, and/or zinc cations from autodeposition baths.
2. Statement of Related Art
Autodeposition involves the use of an aqueous resinous coating composition of relatively low solids concentration (usually less than about 10%) to form a coating of relatively high solids concentration (usually greater than about 10%) on a metallic surface immersed therein, with the coating increasing in thickness and areal density (i.e., mass per unit area of coating) the longer the time the metallic surface is immersed in the composition. Autodeposition is somewhat similar in its results to electrodeposition, but autodeposition does not require the aid of external electrical current to cause the resin particles to deposit on the metal surface.
In general, autodepositing compositions are aqueous acid solutions having solid resin particles dispersed therein in very finely divided form. The coating formed while the metal substrate being coated is immersed in the bath is generally wet and fairly weak, although sufficiently strong to maintain itself against gravity and moderate spraying forces. In this state the coating is described as "uncured". To make an object coated by autodeposition suitable for normal practical use, the uncured coating is dried, usually with the aid of heat. The coating is then described as "cured".
Basic constituents of an autodepositing composition are water, resin solids dispersed in the aqueous medium of the composition, and activator, that is, an ingredient or ingredients which convert the composition into one which will form on a metallic surface a resinous coating which increases in thickness or areal density as long as the surface is immersed in the composition. Various types of activators or activating systems are known. The activating system generally comprises an acidic oxidizing system, for example: hydrogen peroxide and HF; HNO.sub.3 ; a ferric ion containing compound and HF; and other combinations of (i) soluble metal containing compounds such as, for example, silver fluoride, ferrous oxide, cupric sulfate, cobaltous nitrate, silver acetate, ferrous phosphate, chromium fluoride, cadmium fluoride, stannous fluoride, lead dioxide, and silver nitrate, in an amount between about 0.025 and about 50 grams per liter (hereinafter often abbreviated as "g/L"), with (ii) one or more acids such as hydrofluoric, sulfuric, hydrochloric, nitric, and phosphoric acids and organic acids such as, for example, acetic, chloroacetic, and trichloroacetic acids.
The use of autodeposition to coat metal objects containing iron, chromium, and/or zinc causes some dissolution of the objects and therefore increases the concentrations of one or more of these ions in the coating bath. Such increased concentrations of these ions, if sufficiently large, cause the baths to produce unsatisfactory coatings or even to coagulate and thereby become unsuitable for continued use. Removal of these accumulating metal ions is therefore necessary to permit prolonged satisfactory use of an autodeposition bath.
U.S. Pat. No. 3,839,097 of Oct. 1, 1974 to Hall et al. teaches the stabilization of autodeposition baths by removing metal ions therefrom with an ion exchange resin, then regenerating the ion exchange resin by treating it with an aqueous solution of a strong acid. The entire specification of this patent, to the extent not inconsistent with any explicit statement herein, is hereby incorporated into this specification by reference. Sulfuric, phosphoric, hydrochloric, and nitric acids are specifically recommended in this reference for regenerating the ion exchange resins, with 20% by weight sulfuric acid apparently highly preferred, in view of its use in all the examples in which regeneration is described. Similarly, although several types of ion exchange resins are taught by this reference as suitable, only sulfonic acid type resins are used in working examples. This reference teaches that removal of metal cations from an autodeposition bath by use of a cation exchange resin should be supplemented by addition to the bath of dispersing agent(s) selected from the group consisting of cationic and amphoteric surfactants and protective colloids, in order to maintain long term stability and avoid the development of a grainy or textured appearance of the coatings formed by the bath on metal substrates after the bath has been in contact with a cation exchange resin. (The surfactants normally used to stabilize the coating resins in aqueous dispersion in freshly prepared autodeposition baths, in contrast, have always or almost always been anionic surfactants for all autodeposition compositions of practical interest.)
The general use of ion exchange resins has been reviewed by R. E. Anderson in Section 1.12, "Ion exchange Separations", in P. A. Schweitzer (ed.), Handbook of Separation Techniques for Chemical Engineers (McGraw-Hill, New York, 1979). Iminodiacetate resins in particular are described on pages 1-384--1-385 in this reference.
A published product Bulletin, Ion Exchange Resins AMBERLITE.RTM. IRC-718 (Rohm & Haas Co., Philadelphia, 1988), is believed to be typical of current manufacturers' recommendations for use and regeneration of commercial iminodiacetate functional resins. This shows that, at pH 2, iron(III) cations are the most tightly bound to this resin among all commonly occurring cations and states that, for regeneration, "the amount of acid required is higher than that required for conventional weakly acidic ion exchange resins. A regeneration level of 6 to 10 lbs. HCl/ft.sup.3 may be sufficient for metals with moderate selectivity, but this should be increased slightly for tightly held metals. . . . Acid concentration should be 5 to 15 percent, with higher concentrations needed for more tightly bound species."
Published Canadian Patent Application 2,017,026 describes extraction of iron and other metals from electrodeposition baths using iminodiacetate type ion exchange resins. This reference teaches (page 5) that the resins may be regenerated with 20% by weight sulfuric acid solution in water, but otherwise devotes little attention to the regeneration step.
U.S. Pat. No. 4,303,704 of Dec. 1, 1981 to Courduvelis et al. teaches removing complexed copper or nickel from aqueous solutions by passage through a bed of--iminodiacetate type ion exchange resin. This teaches that, "Preferably, a 0.5 to 20% solution of sulfuric acid or other strong acid is used as the eluent."
Japanese Laid Open Application No. 54-24,283, according to an abstract thereof, teaches regenerating ion exchange resins suitable for removing iron compounds from aqueous solutions, using as regenerant an aqueous solution of an aminopolycarboxylic acid, such as hydroxyethylethylene-diaminetriacetic acid, nitrilotriacetic acid, cyclohexanediaminetetraacetic acid, or a water soluble salt of such an acid.